1) Creating the legislation/framework which is to say put into place the structure about how people can be found complicit and sanctions based on various schemes.
2) Outreach and awareness to the at risk population that help is out there and that the behavior is unacceptable and that something other than a scolding is at risk here.
3) Civil actions in high profile cases which reinforce the message and provide case law to guide other actions.
An analog might be civil actions against bars for serving alcohol to clearly inebriated patrons. We did a booth where we sold beer at an Art & Wine festival and had to get mandatory training on the signs so that we wouldn't do something that made us liable under California's laws if we served drunk people and they later did something bad.
But before we got to the 'how' we should probably make sure that we agree that this a parenting problem. In my day reporting bullying to parents was often met with a shrug rather than action, and school expulsion a temporary set back for bullies.